Composite member its separation method and preparation method of semiconductor substrate by utilization thereof

ABSTRACT

In order to separate first and second base substrate without cracking them, and use a damaged base substrate again as a semiconductor substrate to enhance a yield, there is disclosed a preparation method of a semiconductor substrate comprising the steps of separating a composite member formed by bonding the first and second base substrates to each other via an insulating layer into a plurality of members at a separation area formed in a position different from a bonded face to transfer a part of one base substrate onto the other base. A mechanical strength of the separation area is non-uniform along the bonded face in the composite member.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a composite member, its separationmethod and a preparation method of a semiconductor substrate,particularly to a composite member having inside a fragile structurewith a low mechanical strength, its separation method and a preparationmethod of a semiconductor substrate. The present invention isparticularly suitable for a preparation method of a substrate (SOIsubstrate) having an SOI (semiconductor on insulator) structure as atype of semiconductor substrate.

2. Related Background Art

A device using the SOI substrate has various advantages which cannot beattained by an ordinary Si substrate. For example, the advantages are asfollows:

(1) a dielectric is easily separated, and the device is suitable forhigh integration;

(2) the device is superior in resistance to rays;

(3) a floating capacity is small, and high speed operation of elementsis realized;

(4) a well process is unnecessary;

(5) latch-up can be prevented; and

(6) a complete depletion type field-effect transistor can be formed bythinning films.

Since the SOI structure has various advantages as described above,researches concerning its forming method have been advanced theseseveral decades. Known as a conventional SOI technique is an SOS(silicon on sapphire) technique for forming Si on a single-crystalsapphire substrate by hetero epitaxial growth in CVD (chemical vapordeposition) method. The SOS technique has been evaluated as most maturedSOI technique, but has not been put to practical use because of a largeamount of crystal defects caused by lattice mismatching in an interfaceof an SI layer and a base sapphire substrate, mixture of aluminumconstituting the sapphire substrate into the Si layer, substrate price,delay in area enlargement, and for other reasons.

Following the SOS technique, an SIMOX (separation by ion implantedoxygen) technique has appeared. Concerning the SIMOX technique, variousmethods have been developed aiming at reduction of the crystal defectsor reduction of manufacture cost. Examples of the methods include amethod of injecting oxygen ions to a substrate to form an embedded oxidelayer; a method of bonding two wafers via an oxide film, polishing oretching one of the wafers, and leaving a thin single-crystal Si layer onthe oxide film; a method of implanting hydrogen ions into apredetermined depth from a surface of an Si substrate with an oxide filmformed thereon, bonding with the other substrate, leaving a thinsingle-crystal Si layer on the oxide film by heating or anothertreatment, and peeling off the bonded substrate (other substrate); andthe like.

A new SOI technique has been disclosed in Japanese Patent No. 2,608,351or U.S. Pat. No. 5,371,037. In the technique, a first substrate obtainedby forming a non-porous single-crystal layer on a single-crystalsemiconductor substrate with a porous layer formed thereon is bonded ona second substrate and these substrates are bonded, then unnecessaryportions are removed, so that the non-porous single-crystal layer istransferred to the second substrate. The technique is superior in thatthe SOI layer has a superior film thickness uniformity, a crystal defectdensity of the SOI layer can be reduced, the SOI layer has a goodsurface flatness, a manufacture device with expensive and specialspecifications is unnecessary and that SOI substrates having SOI filmsin the range of about several 10 nm to 10 μm can be manufactured withthe same manufacture device.

Furthermore, the present applicant has disclosed a technique in JapanesePatent Application Laid-open No. 7-302889, in which after first andsecond substrates are bonded, the first substrate is separated from thesecond substrate without being collapsed, a surface of the separatedfirst substrate is then smoothed, and a porous layer is formed thereonagain, so that the first substrate is reused. An example of the proposedmethod will be described with reference to FIGS. 12A to 12C. After asurface layer of a first Si substrate 1001 is made porous to form aporous layer 1002, a single-crystal Si layer 1003 is formed on the layer1002, and the single-crystal Si layer and a main surface of a second Sisubstrate 1004 separate from a first Si base substrate are bonded toeach other via an insulating layer 1005 (FIG. 12A). Thereafter, a waferbonded via the porous layer is divided (FIG. 12B), and the porous Silayer exposed to the surface of a second Si base substrate isselectively removed to form an SOI substrate (FIG. 12C). The first Sisubstrate 1001 can be reused by removing the remaining porous layertherefrom.

In the invention disclosed in Japanese Patent Application Laid-open No.7-302889, the substrate is separated using the property that thestructure of the porous silicon layer is more fragile than a non-poroussilicon. Since the substrate once used in preparation of thesemiconductor substrate can be used again in preparation of thesemiconductor substrate, the cost of the semiconductor substrate caneffectively be reduced. Moreover, in the technique, since the firstsubstrate can be used without being wasted, the manufacture cost canlargely be reduced. Additionally, the manufacture process isadvantageously simple.

Examples of the method for separating the first and second basesubstrates (base plates) include pressurizing, pulling, shearing, wedgeinsertion, thermal treatment, oxidization, vibration application, wirecutting, and the like. Additionally, the present inventors have proposeda separation method in Japanese Patent Application No. 9-75498 or U.S.patent application Ser. No. 047,327 filed on Mar. 25, 1998, in whichfluid is sprayed to a separation area. Gas and/or liquid is used as thefluid, and especially a water jet using a liquid mainly composed ofwater is preferable. In the method, at the time of separating, water notonly cuts a bonded face but also uniformly enters a gap-between thefirst and second bases, so that a relatively uniform separating pressurecan be applied to the entire separation face. Moreover, in the method,different from the case where gas is not used, particles can be washedaway without being scattered. The method is superior in these tworespects to the separation method by the wedge insertion. Especially,when the mechanical strength of the separation area is set lower thanthat of the bonded place, only the fragile portion is ruptured,collapsed or removed by spraying the fluid flow to the separation area,and another strong portion can advantageously be left without beingcollapsed.

However, when the water jet or another fluid is used to separate thebonded composite member by spraying the fluid to a side face of acomposite member, especially around a side face of the separation area,there is a case where the fluid flow cannot easily break or cut theseparation area because the separation area has an excessive strength.In this case, the composite member can be separated by raising a fluidpressure, but if the pressure is excessively raised, cracks advanceinward from the side face of the bonded base. In the midst, one or bothof the separated base substrates may be cracked by the pressure of thefluid injected to the separation area. Therefore, yield is lowered inthe separating process. To avoid this, there is provided a method offurther lowering the mechanical strength of the separation area to forma more fragile structure. If the structure is excessively fragile,however, a problem is caused that the separation area is broken andcannot be bonded or that the separation area is broken to generateparticles as contaminants during heating, cleaning, or handling the basesubstrate otherwise in a process of preparing the composite member.

Moreover, when separation is performed in another method without usingthe fluid, basically the similar problem is caused. Therefore, the yieldin the separating process may be lowered.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a composite member andits separation method in which the composite member can be separatedrelatively easily without damaging separated bases.

Another object of the present invention is to provide a composite memberand its separation method in which a mechanical strength of a majorseparation area can be relatively raised, unintended collapse of theseparation area is prevented, and generation of particles is suppressed.

According to an aspect of the present invention, there is provided apreparation method of a semiconductor substrate using a separationmethod of a composite member comprising the step of separating thecomposite member into a plurality of members at a separation area, inwhich a mechanical strength of the separation area is non-uniform alonga bonded face.

Especially, in the separation area, a peripheral portion of thecomposite member is preferably lower in mechanical strength than acentral portion. Additionally, the separation area is preferably lowerin mechanical strength than the bonded interface.

According to another aspect of the present invention, there is provideda preparation method of a semiconductor substrate using the separationmethod mentioned above.

According to still another aspect of the present invention, there isprovided a preparation method of a semiconductor substrate whichcomprises separating a composite member formed by bonding a first basesubstrate and a second base substrate to each other into a plurality ofmembers at a separation area formed in a position different from abonded face, a mechanical strength of the separation area beingnon-uniform along the bonded face, and a mechanical strength of aperipheral portion of the separation area being locally low.

According to a further aspect of the present invention, there isprovided a composite member comprising a separation area inside, amechanical strength of the separation area being non-uniform along asurface of the composite member, a mechanical strength of a peripheralportion of the separation area being locally low.

For the separation area, a porous layer formed by anodization, a layerformed by implanting ions in which microcavities can be obtained, or thelike can be used. When an Si wafer or another semiconductor substrate,or a quartz wafer is used as a first or second base, it substantiallyhas a disc shape although it has an orientation flat or a notch.Therefore, the composite member obtained by bonding the first and secondbase substrates to each other also has a substantially disc shape. Inthis case, when the mechanical strength of the separation area isnon-uniform in such a manner that the strength is high in the centralportion of the composite member and low in its peripheral portion, andsubstantially uniform in a circumferential direction, the compositemember can effectively be separated. When the composite member is arectangular plate member, the mechanical strength of its corner, side orentire periphery is lowered.

The mechanical strength can be made non-uniform by forming portionsdifferent in porosity from one another in the separation area. As theporosity is increased, the mechanical strength is lowered. Therefore,the mechanical strength can be changed by changing the porosity.Specifically, the mechanical strength of the peripheral portion can belowered by setting higher the porosity in the peripheral portion than inthe central portion.

The mechanical strength can also be made non-uniform by changing athickness of the separation area. As the thickness of the separationarea is increased, the mechanical strength is lowered. Therefore, themechanical strength is also changed by changing the thickness.Specifically, the mechanical strength of the peripheral portion can belowered by setting a thickness of a porous layer of the separation arealarger in the peripheral portion than in the central portion of thebase.

In order to obtain a suitable composite member which fails to beseparated in a process prior to a process of separating the compositemember and is securely separated in the separation process, theseparation area is preferably formed by a plurality of layers differentin mechanical strength. Especially, in the separation area comprisingthe plurality of layers, the thickness of a layer high in porosity ispreferably less than the thickness of a layer low in porosity adjacentto a non-porous single-crystal semiconductor layer. A structure of eachof the plurality of layers does not necessarily have to be steeplychanged in an interface. Even if the strength or structure of each layeris continuously changed in the interface of the adjacent layers,separation is facilitated as compared with when the strength is uniformover the entire separation area.

In the separation area comprising the plurality of layers different inthe mechanical strength, the layer high in porosity preferably has ahigher porosity in the peripheral portion than in the vicinity of thecentral portion of the base.

In the separation area comprising the plurality of layers different inthe mechanical strength, the porosity of a second layer with a highporosity can be made higher in the peripheral portion than in thecentral portion of the base substrate by making the thickness of a firstlayer with a small porosity larger in the peripheral portion than in thecentral portion of the base.

The present inventors have conducted experiments in which an anodizationdevice is variously modified to form a good-quality porous layer. As aresult, they have found that there is an Si wafer having an in-planeporosity distribution among a plurality of Si wafers subjected to aporous treatment using a certain mode of anodization device. Moreover,as a result of experiments in which samples are prepared by formingnon-porous layers on porous layers and the non-porous layers are peeledoff, it has been found that in some of the samples even the porous layerrelatively low in porosity can be peeled off more easily than the layerrelatively high in porosity. It is seen from the aforementioned twofindings that, as in an embodiment described later, when the layerrelatively high in porosity is ruptured or collapsed in the porouslayers having the in-plane distribution of porosity, the layerrelatively low in porosity is also easily ruptured, which is not muchinfluenced by an absolute value of porosity.

Specifically, it has been found that when there is a layer relativelyhigh in porosity in the peripheral portion of a member in whichseparation can easily be started, the separation is facilitatedregardless of the absolute value of porosity, and the present inventionhas been developed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C are schematic sectional views of a composite memberaccording to the present invention.

FIGS. 2A and 2B are top views of the composite member according to thepresent invention.

FIGS. 3A and 3B show in-plane distributions of mechanical strength ofthe composite member according to the present invention.

FIG. 4 is view showing a state in which an anodization for use in thepresent invention is applied.

FIG. 5 is a characteristic diagram of porosity of a semiconductorsubstrate according to the present invention.

FIGS. 6A, 6B and 6C are views showing a separation method of thecomposite member according to the present invention.

FIG. 7 is a schematic view of a water jet device.

FIG. 8 is a sectional view of the composite member of the presentinvention.

FIG. 9 is a characteristic diagram showing a porous thickness and ananodization current.

FIG. 10 is a characteristic diagram of porosity of a second layerrelative to thickness of a first layer.

FIG. 11 is a sectional view of the composite member according to anembodiment of the present invention.

FIGS. 12A, 12B and 12C are views showing a conventional preparationmethod of a semiconductor substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1A to 1C are sectional views of a composite member according toone embodiment of the present invention.

The composite member is formed by bonding a first base substrate 1 and asecond base substrate 2 to each other, and a separation area 3 is formedinside. Here, the first base substrate 1 is bonded in such a manner thata layer 4 formed on the separation area 3 abuts on a surface of thesecond base substrate 2, and a bonded interface 5 is formed. Theseparation area 3 has a portion 31 relatively high in mechanicalstrength and a weak portion 32, and the mechanically weak portion 32 ispositioned in a peripheral portion of the composite member (peripheralportion of the separation area). In the case of separating the compositemember, since the portion 32 relatively low in mechanical strength ispositioned in the peripheral portion of the composite member, theportion 32 is first cracked or collapsed, and the separation of thecomposite member is thus facilitated.

More specifically, FIG. 1A shows that the portion 32 of a porousmaterial high in porosity is formed in the peripheral portion of theseparation area 3 uniform in thickness, while the portion 31 of a porousmaterial low in porosity is formed in a central portion, so that theportion 32 locally low in mechanical strength is provided in theperipheral portion. FIG. 2A shows positions of the mechanically strongand weak portions 31, 32 in the composite member as viewed from top.Numeral 7 denotes an orientation flat provided as required.Additionally, as shown in FIG. 2B, the mechanically weak portion 32 maybe partially formed in the outer periphery of the composite member,instead of being formed in the entire outer periphery. The mechanicallystrong portion is larger than the weak portion. FIG. 1B shows themechanically weak portion 32 formed in the peripheral portion by makingnon-uniform the thickness of the separation area 3 of a porous materialuniform in porosity. Also in this case, as shown in FIG. 2B, the portion32 may partially be formed in the outer periphery in the plane of theseparation area 3. FIG. 1C shows the mechanically weak portion 32 formedby implanting ions to form a portion having a large ion implantationamount in the peripheral portion. Also in this case, as shown in FIG.2B, the mechanically weak portion 32 may partially be formed in theouter periphery by locally increasing the ion implantation amount. Whenhydrogen ions or rare gas ions are implanted, and a predeterminedthermal treatment is performed, microcavities are generated. Therefore,a portion in which the ions are injected in high concentration may beformed into a porous portion high in porosity. The mechanically weakportion 32 may locally be formed by setting higher the porosity andthickness of the porous material than the other portions. Moreover, themechanical strength of the ion implantated portion may be lowered bylocally implanting the ions into the separation area formed of theporous material and making fragile the porous material. That is, it ispreferable to appropriately combine the characteristics of thestructures shown in FIGS. 1A to 1C.

Preferably used as the first base substrate 1 of the present inventionis an Si wafer or a plate-like semiconductor wafer of Ge, SiGe, SiC,GaAs, GaAlAs, InP, GaN or the like.

In addition to the same semiconductor wafer as that of the first basesubstrate 1, a quartz glass, resin sheet or another insulating base, anda stainless steel or another metallic base substrate may be used as thesecond base substrate 2.

The non-porous layer preferably comprises a single layer or a pluralityof layers formed of a material selected from the group consisting of thesame semiconductor materials as those for use in the first base. Whenthe composite member is separated to prepare SOI substrate, asingle-crystal semiconductor layer is preferable.

A layer 6 is preferably formed of an insulating material, a conductivematerial, or another material different from the material of the layer4.

Furthermore, the first and second base substrates are preferably bondedvia an insulating layer or an adhesive layer.

FIGS. 3A and 3B are graphs relatively showing distributions ofmechanical strength in the plane of the composite member.

A solid line 10 shows a mode in which the mechanical strength graduallyincreases from a left edge of an outer periphery LE1 toward a center 0of the composite member, and a portion between positions LE2 and RE2including the center 0 has the lowest mechanical strength. A dashed line11 shows a mode in which the mechanical strength has an intermittenttransition between an outer periphery (between outer peripheral edge LE1and position LE2, between outer peripheral edge RE1 and position RE2)and a central portion (from position LE2 to RE2). A broken line 12 showsa mode in which the mechanical strength continuously increases from theouter peripheral edge LE1, RE1 toward the center 0, and the mechanicalstrength takes a maximum value only at the center 0.

In the present invention, it is preferable that the mechanical strengthfrom a position 5 mm inside the outer peripheral edge of compositemember to the outer peripheral edge of the separation area be locallylower than the mechanical strength in the central portion. Referring toFIG. 3A, the separation area is preferably formed as a thin layer insuch a manner that the position 5 mm inside the outer peripheral edge ofthe composite member is between LE1 and LE2 and/or between RE2 and RE1.

Furthermore, when a large-diameter composite member is separated fromits outer peripheral edge toward the center, there is a case where thecentral portion of the composite member cannot be separated as desired.In this case, a mechanically weak portion may locally be formed in thecentral portion. FIG. 3B shows an example of such mode, in which aportion between periphery and center, i.e., a donut-shaped portion M ishigh in mechanical strength.

When a porous layer is used as the separation area, the porosity of themechanically weak peripheral portion is set to 20% or more, preferably35% or more, and the upper limit of the porosity may be 80% or less.

The porosity of the mechanically strong central portion is not limitedas long as it is lower than that of the peripheral portion, but may beselected preferably from the range of 5% to 35%, more preferably fromthe range of 5% to 20% in such a manner that the porosity becomes lowerthan that of the peripheral portion. When a difference in porosity is 5%or more, preferably 10% or more, a difference of mechanical strengthsufficient for easily separating the composite member can be obtained inthe peripheral portion and the central portion.

Additionally, in FIG. 3B, the portions M are mechanically strong.Therefore, when the separation area is formed of the porous material,the porosity of the portion M, i.e., the portion having the maximumvalue of mechanical strength may be set low in the range of 5% to 35%,preferably 5% to 20%, in the same manner as the porosity of the centralportion of FIG. 3A.

The porosity of the center 0 in FIG. 3B needs to be higher than that ofthe portion M, and may appropriately be selected from the range of 20%to 80% to satisfy such relationship.

Here, the porosity P (%) of the porous material indicates a proportionof pore volume in an apparent volume of the porous material. Theporosity is represented in the following equation using a density m ofthe porous material formed on the first base substrate and a density Mof a non-porous material:

P={(M−m)÷M}×100(%)  (1)

Here, the density m of the porous material is obtained by dividing anapparent weight G of the porous material including pores by an apparentvolume V of the porous material including the pores, and represented asfollows:

m=G÷V  (2)

In practice, the porosity P of the porous layer of the base substrate inwhich only a depth d on the side of a surface has a porous layerstructure can be obtained from the following equation using a weight Aof the base substrate before the porous layer is formed thereon, aweight a of the base substrate after the porous layer is formed thereon,and a weight B of the base substrate after the porous layer iscompletely removed therefrom:

P={(A−a)÷(A−B)}×100  (3)

A method of preparing the composite member will next be described.

First, the first base substrate 1 of an Si wafer or the like isprepared, and the separation area 3 is formed on the surface of thefirst base substrate 1 or at a predetermined depth from the surface.Examples of the method for forming the separation area include a methodof making the surface of the first base substrate 1 porous byanodization or the like and/or a method of implanting hydrogen ions,rare gas ions, or the like different from constituting elements of thebase substrate to form an ion-implanted layer with a maximum ionimplantation concentration at the predetermined depth from the surfaceof the first base substrate 1. The mechanically weak portion is formedin the peripheral portion by controlling the conditions of theanodization or ion implanting with procedures described later.

Subsequently, the non-porous layer 4 is formed on the separation area 3if necessary, and the second base substrate is bonded. In the case ofimplanting the ions, a surface layer of the first base substrateconstitutes the non-porous layer 4 as it is. When the surface of thefirst base substrate 1 is made porous, the layer 4 is formed thereon bysputtering or CVD. Subsequently, the non-porous layer 4 is bonded on thesecond base substrate of Si wafer or the like directly or via theinsulating layer 6 as required. The composite member is thus completed.

In a method for forming the porous layer locally low in mechanicalstrength, the current density of anodization is changed in the plane.When the density of anodization current flowing into the peripheralportion of the semiconductor substrate is set high in the peripheralportion of the base, the thickness and/or porosity in the peripheralportion of the base substrate of the porous layer can be made higherthan in the central portion of the base. To realize the current densitydistribution, for example, during the anodization, a sectional area, inwhich ion current flows, in an anodization liquid in the vicinity of thebase substrate subjected to the formation is set larger than an area ofthe base substrate subjected to the formation. Thereby, a surfacedensity of anodization current flowing into the base substrateperipheral portion may be set higher than a surface density ofanodization current flowing into the base substrate center.Specifically, an anodization bath larger than the base substratesubjected to the formation is used, so that the ion current having asectional area broader than the area of the base substrate is receivedby the base.

FIG. 4 is a schematic view showing a device for use in anodization. InFIG. 4, numeral 101 denotes a DC power supply for the anodization, 102denotes a cathode electrode, 103 denotes an anode electrode, and 104,105 denote insulating supports for supporting the treated base substrate1. The base substrate 1 is engaged in recesses of the supports 104, 105.Numeral 106 denotes an insulating bath bottom. The area of the electrode102, 103 is about 1.2 to 3.0 times, preferably 1.3 to 2.0 times the areaof the first base substrate 1. In the structure, when ions flowing fromthe outside via the outer peripheral edge of the base substrate arecollected in the base, more ions flow into the peripheral portion of thebase, and the thickness and porosity of the porous layer of theperipheral portion can be raised.

Furthermore, while a plurality of stages of anodization are performed,the peripheral portion of the first porous layer is formed thicker thanthe central portion. Therefore, the porosity of the peripheral portionof the second porous layer formed later can be made higher than theporosity of the central portion.

When such distribution of flowing currents needs to be controlled moreprecisely, a current guide is provided in the vicinity of the basesubstrate subjected to the formation for controlling the distribution ofion currents flowing into the base substrate surface. When the ioncurrent distribution is controlled, a distribution of thickness of thelayer with a small porosity can be controlled.

When the layer formed by the ion implanting in which microcavities canbe obtained is used as the separation area, the size or density ofmicrocavities or the thickness of distributed microcavities can beincreased by raising an ion implanting density, so that the mechanicalstrength of the area can be reduced.

Therefore, when the ion implantation amount of the base substrateperipheral portion is set larger than that of the base substrate centralportion, the density of microcavities per unit volume of the basesubstrate peripheral portion is raised, and the porosity can be madehigher than that of the base substrate central portion.

FIG. 5 is a graph showing an in-plane distribution of porosity in adiametric direction of the porous material obtained in the method shownin FIG. 4.

As the porosity increases, the mechanical strength decreases. Therefore,FIG. 5 shows a pattern vertically reverse to the pattern shown by thesolid line 10 of FIG. 3A. When the ratio of the area of the electrode102, 103 to the area of the base substrate is sufficiently large, apattern shown by a solid line 15 is provided. When the ratio of the areaof the electrode to the area of the base substrate is small, a tendencyshown by a broken line 14 is provided. The porous material with a highporosity can thus be formed in the peripheral portion.

A technique for preparing a porous layer with a mechanical strengthdistribution shown in FIG. 1A or by the dashed line 11 of FIG. 3A willnext be described. A first method is as follows:

A mask for ion implantation or photoresist pattern is provided only onthe outer peripheral portion of the base substrate 1, while boron ionsare injected to the central portion. The base substrate having a lowboron ion concentration locally in the outer periphery is subjected toanodization using an electrode having substantially the same area asthat of the base, and the porous layer is prepared in which the outerperipheral portion has a high porosity while the central portion has alow porosity.

A second method is as follows:

The central portion of the base substrate excluding its outer peripheralportion is covered with wax or another mask resistant to theanodization, and general anodization is performed under a high currentdensity to make porous the outer peripheral portion. Subsequently, theouter peripheral portion is masked, and the general anodization isperformed under a low current density to make porous the centralportion.

In a third method, after a uniform porous layer is formed by the generalanodization, only the porosity of the outer peripheral portion is raisedby ion implanting. When the distribution of ion implantation amount iscontrolled, the porous layer having the strength distribution as shownin FIGS. 3A, 3B can be formed with good controllability.

Additionally, in respect of the manufacture cost, the method shown inFIG. 4 is more advantageous than these methods.

A method of preparing the composite member shown in FIG. 1C will next bedescribed in more detail.

An Si wafer or another base substrate is oxidized to form the insulatingfilm 6. Hydrogen or rare gas ions are implanted into the entire surfaceof the base substrate with a predetermined acceleration voltage. Thecentral portion excluding the outer peripheral portion is covered with aphotoresist mask pattern, and the ions are again implanted into theouter peripheral portion with the same acceleration voltage. Theseparation area 3 having the mechanically weak portion 32 can be formedin this manner. After the mask pattern is removed, the insulating layer6 is bonded onto the second base substrate 2. Each of doses in first andsecond ion implanting operations is set in the range of 10¹⁵ cm⁻² to10¹⁷ cm⁻², while a concentration of different atoms in the mechanicallyweak portion may be set in the range of 10²⁰ cm⁻³ to 10²³ cm⁻³.

A method of separating the composite member for use in the presentinvention will next be described. A mode of separating the compositemember shown in FIG. 1A will be described as an example. As shown inFIG. 6A, an inner stress generated by thermal treatment or the like oran external force is used to separate the composite member. In theseparation area 3, since the peripheral portion 32 locally has a lowmechanical strength, it is first collapsed or cracked. FIG. 6A showsthat a wedge 110 is inserted and a force 111 is applied to separate theperipheral portion of the first base substrate 1 from the second basesubstrate 2. Subsequently, as shown in FIG. 6B, the composite member isdivided into two. When a residual layer 37 of the separation area 3remaining on the non-porous layer 4 is relatively thick, the residuallayer is removed by polishing or etching. Subsequently, thermaltreatment (hydrogen annealing) is performed in an atmosphere of hydrogenif necessary. As shown in FIG. 6C, the base substrate 2 provided withthe layer 4 having a smooth surface is obtained. For use in a solarbattery, the residual layer does not need to be removed.

Examples of the separation method of the composite member usable in thepresent invention include pressurizing, pulling, shearing, wedgeinsertion, thermal treatment, vibration application, wire cutting, andvarious methods as disclosed in Japanese Patent Application Laid-openNo. 7-302889. Additionally, as proposed in Japanese Patent ApplicationNo. 9-75498, the bonded first and second base substrates may beseparated into a plurality of members in a separation area other than abonded interface by spraying fluid or ejecting a jet of fluid to thevicinity of a side face of the separation area.

The jet of fluid for use in separation in the present invention can berealized by spouting pressurized fluid via a thin nozzle. A fluid jetmethod, as introduced in “Water Jet” Vol. 1, No. 1, page 4, can be usedas a method for spouting a high-speed high-pressure flow of beams. Inthe fluid jet usable in the present invention, a liquid with a highpressure in the range of 100 to 3000 kgf/cm² pressurized by ahigh-pressure pump is spouted via a fine nozzle with a diameter of about0.1 to 0.5 mm, so that ceramic, metal, concrete, resin, rubber, wood oranother material can be cut (abrasive is added to water for a hardmaterial), or worked. Additionally, a coating film of a surface layercan be removed, or member surfaces can be cleaned. In the conventionalwater jet method, it is a main effect to remove a part of the materialas described above. Specifically, in the water jet cutting operation, acutting width of a main member is removed, coating films are removed, ormember surfaces are cleaned by removing unnecessary portions.

When water jet is used to form the fluid flow according to the presentinvention, the composite member can be separated by spouting the waterjet to the side face of the separation area. In this case, the side faceof the separation area is first exposed to the side face of the bondedbase, and the water jet is directly spouted to the exposed portion orits peripheral portion. Then, the base substrate is separated into twowithout being damaged, while only the mechanically fragile separationarea is removed by the water jet. Moreover, even if the side face of theseparation area is not exposed beforehand for some reason, and thecorresponding portion is covered with a thin layer like an oxide film,the layer covering the separation area is first removed with the waterjet, before the base substrate can be separated with the water jet.

Moreover, an unused effect of the conventional water jet is used.Specifically, jet is spouted to a recess in the side face of thecomposite member to extend and collapse the structurally fragileseparation area, so that the bonded wafer can be separated. In thiscase, chips of the separation area are hardly generated. Even if theseparation area is formed of a material which cannot be removed by thejet, separation can be performed without using the abrasive or withoutdamaging a separating surface.

The aforementioned effect is not an effect of cutting or polishing, andit can be expected to be an effect of wedge by the fluid as shown inFIG. 6A. This effect is much expected when a force is applied in adirection in which the separation area is pulled off by spouting the jetto a recess formed in the side face of the bonded base. In order tosufficiently fulfill the effect, the shape of the side face of thecomposite member is preferably concave, instead of being convex.

FIG. 7 is a schematic perspective view showing an example of a water jetdevice for use in the method of manufacturing the semiconductorsubstrate in the present invention. In FIG. 7, a composite member 1 isformed by integrally bonding two Si wafers, and the separation area 3 isprovided inside. Supports 403, 404 are provided on the same rotatingshaft for adsorbing/fixing the composite member 1 by a vacuum chuck.Furthermore, the support 404 is connected to a support base substrate409 via a bearing 408, and its rear is directly connected to a speedcontrol motor 410, so that the support 404 can be rotated at anarbitrary speed. Moreover, the support 403 is connected to the supportbase substrate 409 via a bearing 411, and its rear is connected to thesupport base substrate 409 via a compression spring 412, so that a forceis applied in a direction in which the support 403 is detached from thecomposite member 1.

First, the composite member 1 is set in accordance with a positioningpin 413, and adsorbed/held by the support 404. Since the compositemember 1 is positioned by the positioning pin 413 of a tool 407, thecentral portion of the composite member 1 can be held. Subsequently, thesupport 403 is advanced toward the left along the bearing 411 until thecomposite member 1 is adsorbed/held. Then, a force exerted toward theright is applied to the support 403 by the compression spring 412. Inthis case, a returning force of the compression spring 412 and a forceof the support 403 for sucking the composite member 1 are balanced toprevent the support 403 from being detached from the composite member 1by the force of the compression spring 412.

Subsequently, water is fed to a water jet nozzle 402 from a water jetpump 414, and water is continuously spouted for a constant time untilspouted water is stabilized. When the water is stabilized, a shutter 406is opened to spout the water (hereinafter referred to as the jet water)to the side face of the composite member 1 from the water jet nozzle402. In this case, the composite member 1 and the support 403 arerotated by rotating the support 404. Since the jet water is applied nearthe center of the thickness of the side face of the composite member 1,the composite member 1 is pushed/extended into two toward its centerfrom its outer peripheral portion, the separation area relatively weakin the composite member 1 is collapsed, and the composite member isfinally separated into two pieces.

As described above, the jet water is applied uniformly to the compositemember 1. Moreover, while the support 403 supports the composite member1, the force is exerted toward the right. Therefore, the separatedpieces of the composite member 1 do not slide on each other.

Alcohol or another organic solvent; hydrofluoric acid, nitric acid oranother acid; potassium hydroxide or another alkali; or another liquidhaving a function of selectively etching the separation area can be usedas the fluid instead of water. Furthermore, usable as the fluid is air,nitrogen gas, carbonic acid gas, rare gas or another gas. Gas or plasmahaving a function of etching the separation area can also be used. Inthe separation method of the composite member introduced in thepreparation method of the semiconductor substrate, pure water from whichimpurity metals or particles are removed as much as possible, super purewater or another water with high purity is preferably used. Moreover, acompletely low-temperature process is introduced. Therefore, even if thejet fluid other than pure water is used, the impurities or particles canbe removed by cleaning after the separation.

In the method of spraying the fluid as described above, the vicinity ofthe separation area of the composite member is preferably recessed in aconcave shape for receiving the fluid to produce a force in a directionin which the separation area is pushed/extended. When the compositemember formed by bonding two base substrates via the separation area isseparated at the separation area, the aforementioned structure caneasily be realized by chamfering edges of the bases.

The water jet or another fluid flow, pressurizing, pulling, shearing,wedge insertion, thermal treatment, vibration application, wire cutting,and other various methods may be used to apply a separating force to theseparation area formed beforehand in the composite member and toseparate the composite member into two. In this case, the separation isperformed by collapsing the mechanically fragile portion of theseparation area. When the fluid is spouted to the vicinity of theseparation area, the mechanically fragile separation area is removed orcollapsed by the fluid flow. When the fluid is used, however, basicallythe separation area is removed while the other non-fragile portionsremain without being collapsed. As a result, the separation canadvantageously be performed without damaging any portion that is usedafter the separation. In any of the methods, however, unless theseparation area is sufficiently weak, it cannot be collapsed. Forexample, there is a case where the separation area cannot be collapsedor removed with the fluid flow with a predetermined pressure.

To solve the problem, when the pressure of the fluid is raised, not onlythe separation area but also the other portions are collapsed. Forexample, when the bonded base substrate is separated, the plate-likefirst or second base substrate is cracked. To prevent this, when thepressure of the fluid is lowered, however, the separation cannot beperformed.

In most of the separation methods, in the initial separation stage, asolid wedge needs to be pushed into the vicinity of the surface of theseparation area formed in the composite member, e.g., the separationarea of the portion formed in the peripheral portion of the disc-shapedbonded base. In many cases, the separation needs to proceed from thesurface. While the separation fails to proceed, the portion close to thesurface has a small area to which the separating force is applied. Aproblem is therefore caused that a surface density of force has to beraised. This is because the separating force can be applied to theseparated surface, but no separating force can be applied to the surfacenot yet separated. When the separation proceeds, the area to which theseparating force can be applied is enlarged. Therefore, even when theseparating force applied to the separating surface is increased tofacilitate the separation, the surface density is decreased, and thebase substrate can easily be prevented from being broken (cracked orotherwise) by the separation.

In order to facilitate the separation, in the initial stage ofseparation, the mechanical strength may be lowered by raising theporosity of the porous layer of the separation area, increasing thethickness of the porous layer, or increasing the ion implanting amountto increase the amount of generated microcavities. When the strength isexcessively lowered, however, a disadvantage is caused that in theprocess of forming the composite member, the separation area iscollapsed before the separation process.

As a result of intensive researches, the present inventors have foundthat in order to avoid the aforementioned disadvantages, the mechanicalstrength of the separation area is changed in parallel with the bondedface, and the mechanical strength particularly of the portion of theseparation area close to the bonded base substrate surface, e.g., theperipheral portion is set lower than that of the base substrate centralportion.

In the initial stage of separation, since the area of the separated faceis small and the separating force cannot be increased, the mechanicalstrength of the separation area is reduced, so that the separation isadvanced with a small force. This is realized by reducing the mechanicalstrength of the separation area in the vicinity of the base substrateperipheral portion. Peeling during the process is prevented by settingthe mechanical strength of the separation area higher in the basesubstrate central portion than in the peripheral portion.

In this case, when the separation is advanced to the central portion,the separated area is broad. Therefore, even when the surface density ofthe separating force is reduced, the entire separating force isincreased, and the separation can be advanced. Such effect is fulfilledregardless of the separation method, but the method of spouting thefluid flow to the separation area is most preferable for applying theseparating force relatively uniformly to the entire separated face toprevent the base substrate from being broken.

In order to broaden the range of conditions for performing a stableseparation to securely separate the base substrate without damaging it,as shown in FIG. 8, the separation area 3 is preferably formed of aplurality of layers or areas 22, 23 different in mechanical strength. Inthis case, the mechanical strength of the peripheral portion canrelatively easily be made smaller as compared with that of the centralportion of the base. When the separation area has a lamination structureof the layer 23 small in porosity (hereinafter referred to as the firstporous layer) and the layer 22 large in porosity (hereinafter referredto as the second porous layer), the layer 23 small in porosity is firstformed by the anodization, and subsequently the anodization current isincreased to similarly form the layer 22 large in porosity by theanodization.

As a result of intensive researches, the present researchers have foundthat the porosity of the second porous layer 22 is not determined onlyby the magnitude of the current, and it also depends on the thickness orporosity of the first porous layer 23. When the anodization current ofthe second porous layer 22 is set equal, but the first porous layer 23is thick or low in porosity, then the porosity of the second porouslayer 22 tends to be increased. Therefore, for example, as the thicknessof the first porous layer 23 is reduced, the anodization current of thesecond porous layer 22 needs to be raised in order to keep high theporosity of the second porous layer 22. This relationship is shown inFIG. 9.

If the anodization current of the second porous layer is kept constant,and the thickness of the first porous layer is changed, the porosity ofthe second porous layer is influenced. Such relationship is shown inFIG. 10. It is apparent that after the first porous layer is formed, thesecond porous layer cannot be formed independently, and thecharacteristics of the first porous layer exert an influence on theporosity of the second porous layer. A detailed mechanism of suchphenomenon is not completely explicated. As described later, however, F⁻ions in a formation liquid are necessary for forming porous Si. When theF⁻ ions are consumed in a pore forming portion at a tip end of a pore,new F⁻ ions need to be supplied to the tip end of the pore from asurface of the porous Si through the pore.

It is supposed that such effective transportability of the F⁻ ions inthe pore by electric field or diffusion depends on the pore size orlength of the first layer, i.e., the thickness of the first layer.Specifically, the first porous layer itself formed by the anodizationlimits the transport of ions necessary for forming the subsequent porouslayer.

Therefore, the formed first porous layer serves as a layer for limitingthe effective transportability of the F⁻ ions necessary for forming thesubsequent porous layer. When the anodization current is constant, theformation is advanced to form a sufficient thickness without largelychanging the porosity. This is because a pore of a size determined by abalance between consumption and supply of F⁻ ions is formed at aconstant current, but if the current is increased halfway, the balancebetween consumption and supply of the F⁻ ions is changed by theexistence of the formed porous layer, and the pore size is largelychanged.

When the thickness of the first layer is increased and the effectivetransportability of F⁻ ions transported through the layer is lowered,the concentration of F⁻ ions in the tip end of the pore is decreased,and an ion lacking layer is spread in the formation liquid in the pore.Therefore, a portion in which a potential barrier of an interfacebetween formation liquid and Si single-crystal surface in the pore islowered is extended. In the portion Si is etched, and the pore size maybe increased.

In practice, even when the anodization current is simply increased, theporosity is not much increased unless the transportability limitinglayer is formed on the Si surface. This rather increases the formationrate. Therefore, in order to largely change the porosity by increasingthe anodization current, the layer for limiting the transportability ofthe F⁻ ions is necessary between porosity increasing layer and formationliquid. If the thickness of the first porous layer can be increased inthe periphery of the base, the porosity of the second porous layer inthe corresponding portion can be larger than the porosity of the secondlayer in a central portion where the first layer is thin. Thereby, themechanical strength of the separation area of the base substrateperipheral portion can be reduced.

The present invention is characterized in that when the mechanism of theanodization is well used as described above to form the separation areacomprising a plurality of layers or areas different in the mechanicalstrength, the porosity of the layer 22 large in porosity can be madehigher in the peripheral portion than in the central portion of the basesubstrate by increasing the thickness of the layer 23 small in porosityin the peripheral portion rather than in the central portion of thebase.

As described above, the porous layer can be formed on the wafer by theanodization using the simple device shown in FIG. 4. The layer with asmall porosity can be formed thicker in the base substrate peripheralportion than in the base substrate central portion, which can makehigher the porosity of the subsequently formed layer with a largeporosity in the base substrate peripheral portion than in the basesubstrate central portion. When the distribution of incoming currentneeds to be controlled more precisely, the current guide is provided inthe vicinity of the base substrate subjected to the formation forcontrolling the distribution of ion currents flowing into the basesubstrate surface. When the ion current distribution is controlled, thedistribution of thickness of the layer with a small porosity can becontrolled.

Moreover, the water jet injection device for separating the wafer andthe thin-film semiconductor from the composite member comprising thefirst and second base substrates has been described above with referenceto FIG. 7.

An example of the bonded base substrate usable in the method of thepresent invention will be described in more detail with reference toFIG. 8. In the example, as shown in FIG. 8, the separation area 3 has adouble layer structure comprising the first porous layer 23 with a lowporosity and the second porous layer 22 with a higher porosity and alower mechanical strength. In the present invention, for the secondporous layer 22, its porosity or thickness may be set higher in thevicinity of the base substrate peripheral portion than in the centralportion. During the separation, cracks are generated in the secondporous layer 22 in a position different from the bonded interface or inthe interface. The second porous layer 22 has a low mechanical strength.Therefore, when a force is applied in a direction in which a first basesubstrate 21 and a second base substrate 27 are separated from eachother, only the second porous layer 22 is collapsed and the basesubstrates are separated. In this case, when a layer 4 of non-poroussingle-crystal Si is formed, the first porous layer 23 is necessary as aprotective layer for suppressing the generation of crystal defects orfor preventing the layer 4 from being collapsed in the separationprocess. When the porosity is not much increased, the separation can beperformed without forming the second porous layer 22, but the secondporous layer 22 is preferably formed to provide a good yield.

Embodiments of the present invention will next be described in moredetail.

EXAMPLE 1

A first P-type (or N-type) (100) single-crystal Si substrate with athickness of 625 μm, a specific resistance of 0.01 Ω·cm and a diameterof eight inches was used, and anodization was performed in HF solution.A formation bath was prepared in such a manner that a sectional area ofa plane parallel with a formation electrode of an anodization layer andthe Si single-crystal base substrate was about twice an area of the Sibase, and the formation bath was used.

Anodization conditions are as follows:

anodization current: 2.6 A

anodization solution:HF:H₂O:C₂H₅OH=1:1:1

time: 11 minutes

The thickness of a central portion of a porous layer of the basesubstrate subjected to the formation was about 12 μm and the porosity ofthe central portion was about 20%, while the thickness of the porouslayer of a peripheral portion was about 19 μm at maximum and theporosity was 30%. The pore size of the peripheral portion of the basesubstrate prepared under these conditions can be measured by observationwith an electronic microscope. It is apparent that the pore size islarger in a portion deep from a surface than in the central portion.However, for the central portion or the peripheral portion, noremarkable difference in pore size can be found in the vicinity of thesurface of the porous layer. This is essential for the subsequentprocess, in which Si single crystal having less defects is epitaxiallydeveloped into a porous layer structure.

The substrate was cleaned with hydrofluoric acid at 400° C. in theatmosphere of oxygen, then oxidized for one hour. Inner walls of poresof the porous Si were covered with a thermally oxidized film through theoxidization. After thermal treatment was performed at 950° C. in theatmosphere of hydrogen, single-crystal Si epitaxially grew by 0.3 μm onthe porous Si by CVD method under the following conditions:

source gas: SiH₄

carrier gas: H₂

temperature: 900° C.

pressure: 1×10⁻² Torr

growth rate: 3.3 nm/sec

Furthermore, 100 nm of SiO₂ layer was formed on a surface of theepitaxial Si layer by the thermal oxidization.

After the surface of the SiO₂ layer and a surface of a separatelyprepared Si substrate were overlapped and contacted with each other,thermal treatment was performed at 1180° C. for five minutes to performbonding. When the composite member was set in the device shown in FIG.7, and water jet injection was performed with a water pressure of 1000kgf/cm² and a diameter of 0.15 mm, the porous Si layer was collapsed,the wafer was effectively divided in two, and the porous Si was exposedto a separated face of two Si substrates. Subsequently, the porous Silayer was selectively etched with etching liquid of HF/H₂O₂/C₂H₅OH. Theporous Si was selectively etched and completely removed. The etchingrate of the non-porous Si single crystal to the etching liquid isremarkably low, and the etching amount in the non-porous layer canpractically be ignored. Specifically, a single-crystal Si layer having athickness of 0.2 μm could be formed on the oxidized Si film. Thesingle-crystal Si layer underwent no change even by the selectiveetching of the porous Si. Resulting SOI substrate was thermally treatedin the atmosphere of hydrogen.

As a result of the observation of a cross section by a transmissionelectronic microscope, it was confirmed that no crystal defect wasintroduced to the Si layer and that good crystallizability was kept.Even when no oxide film was formed on the surface of the epitaxial Silayer, similar results were obtained. The first Si single-crystalsubstrate was reused as a first Si single-crystal substrate forobtaining another SOI substrate, by removing residual porous Sitherefrom.

EXAMPLE 2

A first P-type (or N-type) (100) single-crystal Si substrate with athickness of 625 μm, a specific resistance of 0.01 Ω·cm and a diameterof eight inches was used, and anodization was performed in HF solution.A formation bath was prepared in such a manner that a sectional area ofa plane parallel with a formation electrode of an anodization layer andthe Si single-crystal base substrate was about twice an area of the Sibase, and was used.

Anodization conditions are as follows:

anodization current: 2.6 A

anodization solution:HF:H₂O:C₂H₅OH=1:1:1

time: 11 minutes

The thickness of a central portion of a first porous layer of the basesubstrate subjected to the formation was about 12 microns, and theporosity of the central portion was about 20%. The thickness of theporous layer of a peripheral portion was about 19 μm at maximum and theporosity was 30%. Subsequent to the formation of the first layer, theformation of a second layer was performed under the followingconditions:

anodization current: 8 A

anodization solution:HF:H₂O:C₂H₅OH=1:1:1

time: two minutes

When the formation of the second layer was performed under the aboveconditions after the first layer was formed, the thickness of the centerof the second layer was about two microns, and the porosity was about40%. In the peripheral portion of the base, however, the porosity wasabout 55% at maximum, and its thickness was less than two microns.

However, for the central portion or the peripheral portion, noremarkable difference in pore size can be found in the vicinity of thesurface of the first porous layer. This is essential for the subsequentprocess, in which Si single crystal having less defects is epitaxiallydeveloped into a porous layer structure.

The substrate was oxidized at 400° C. in the atmosphere of oxygen forone hour. Inner walls of pores of the porous Si were covered with athermally oxidized film through the oxidization. Subsequently, aftercleaning was performed with HF solution and thermal treatment wasperformed in the atmosphere of hydrogen, single-crystal Si epitaxiallygrew by 0.3 μm on the porous Si by CVD method. The growing conditionswere as follows:

source gas: SiH₄

carrier gas: H₂

temperature: 900° C.

pressure: 1×10⁻² Torr

growth rate: 3.3 nm/sec

Furthermore, 100 nm of SiO₂ layer was formed on a surface of theepitaxial Si layer by the thermal oxidization.

After the surface of the SiO₂ layer and a surface of a separatelyprepared Si substrate were overlapped and contacted with each other,thermal treatment was performed at 1180° C. for five minutes to performbonding. A cross section of resulting composite member isdiagrammatically shown in FIG. 11. The porous layer was exposed to waferedges, the porous Si was etched to some degree, and a plate as sharp asa razor blade was inserted to the corresponding portion. Then, theporous Si layer was ruptured, the wafer was divided into two, and theporous Si was exposed. Subsequently, the porous Si layer was selectivelyetched with etching liquid of HF/H₂O₂/C₂H₅OH. The porous Si wasselectively etched and completely removed. The etching rate of thenon-porous Si single crystal to the etching liquid is remarkably low,and the etching amount in the non-porous layer provides a practicallyignorable decrease of thickness. Specifically, a single-crystal Si layerhaving a thickness of 0.2 μm could be formed on the oxide Si film. Thesingle-crystal Si layer underwent no change even by the selectiveetching of the porous Si. Resulting SOI substrate was thermally treatedin the atmosphere of hydrogen.

As a result of the observation of a cross section by a transmissionelectronic microscope, it was confirmed that no crystal defect wasintroduced to the Si layer and that good crystallizability was kept.Even when no oxidized film was formed on the surface of the epitaxial Silayer, similar results were obtained. The first Si single-crystalsubstrate was reused as a first Si single-crystal substrate by removingresidual porous Si therefrom.

EXAMPLE 3

A first P-type (or N-type) (100) single-crystal Si substrate with athickness of 625 μm, a specific resistance of 0.01 Ω·cm and a diameterof eight inches was used, and anodization was performed in HF solution.A formation bath was prepared in such a manner that a sectional area ofa plane parallel with a formation electrode of an anodization layer andthe Si single-crystal base substrate was about twice an area of the Sibase, and was used.

Anodization conditions are as follows:

anodization current: 2.6 A

anodization solution:HF:H₂O:C₂H₅OH=1:1:1

time: 11 minutes

The thickness of a central portion of a first porous layer of the basesubstrate subjected to the formation was about 12 microns, and theporosity of the central portion was about 20%. The thickness of theporous layer of a peripheral portion was about 19 μm at maximum and theporosity was 30%. Subsequent to the formation of the first layer, theformation of a second layer was performed under the followingconditions:

anodization current: 8 A

anodization solution:HF:H₂O:C₂H₅OH=1:1:1

time: two minutes

When the formation of the second layer was performed under the aboveconditions after the first layer was formed, the thickness of the centerof the second layer was about two microns, and the porosity was about40%. In the peripheral portion of the base, however, the porosity wasabout 55% at maximum, and its thickness was less than two microns.

However, for the central portion or the peripheral portion, noremarkable difference in pore size can be found in the vicinity of thesurface of the first porous layer. This is essential for the subsequentprocess, in which Si single crystal having less defects is epitaxiallydeveloped into a porous layer structure.

The substrate was oxidized at 400° C. in the atmosphere of oxygen forone hour. Inner walls of pores of the porous Si were covered with athermally oxidized film through the oxidization. After cleaning wasperformed with HF solution and thermal treatment was performed in theatmosphere of hydrogen, single-crystal Si epitaxially grew by 0.3 μm onthe porous Si by CVD method. The growing conditions were as follows:

source gas: SiH₄

carrier gas: H₂

temperature: 900° C.

pressure: 1×10⁻² Torr

growth rate: 3.3 nm/sec

Furthermore, 100 nm of SiO₂ layer was formed on a surface of theepitaxial Si layer by the thermal oxidization.

After the surface of the SiO₂ layer and a surface of a separatelyprepared Si substrate were overlapped and contacted with each other,thermal treatment was performed at 1180° C. for five minutes to performbonding. The composite member as shown in FIG. 11 was thus obtained. Forwafer side faces, water jet injection was performed with a waterpressure of 300 kgf/cm² and a diameter of 0.1 mm. Then, the porous Silayer was ruptured, the wafer was effectively divided into two, and theporous Si was exposed. Subsequently, the porous Si layer was selectivelyetched with etching liquid of HF/H₂O₂/C₂H₅OH. The porous Si wasselectively etched and completely removed. The etching rate of thenon-porous Si single crystal to the etching liquid is remarkably low,and the etching amount in the non-porous layer provides a practicallyignorable decrease of thickness. Specifically, a single-crystal Si layerhaving a thickness of 0.2 μm could be formed on the oxide Si film. Thesingle-crystal Si layer underwent no change even by the selectiveetching of the porous Si. Resulting SOI substrate was thermally treatedin the atmosphere of hydrogen.

As a result of the observation of a cross section by a transmissionelectronic microscope, it was confirmed that no crystal defect wasintroduced to the Si layer and that good crystallizability was kept.Even when no oxidized film was formed on the surface of the epitaxial Silayer, similar results were obtained. The first Si single-crystalsubstrate was reused as a first Si single-crystal substrate by removingresidual porous Si therefrom.

EXAMPLE 4

A first P-type or N-type (100) single-crystal Si substrate with athickness of 625 μm, a specific resistance of 0.01 Ω·cm and a diameterof eight inches was used, and anodization was performed in HF solution.A formation bath was prepared in such a manner that a sectional area ofa plane parallel with a formation electrode of an anodization layer andthe Si single-crystal base substrate was about 1.3 times an area of theSi base, and was used.

Anodization conditions are as follows:

anodization current: 2.6 A

anodization solution:HF:H₂O:C₂H₅OH=1:1:1

time: 11 minutes

The thickness of a central portion of a first porous layer of the basesubstrate subjected to the formation was about six microns, and theporosity of the central portion was about 20%. The thickness of theporous layer of a peripheral portion was about eight μm at maximum andthe porosity was 25%. Subsequent to the formation of the first layer,the formation of a second layer was performed under the followingconditions:

anodization current: 12 A

anodization solution:HF:H₂O:C₂H₅OH=1:1:1

time: one minute

For the central portion or the peripheral portion, no remarkabledifference in pore size can be found in the vicinity of the surface ofthe first porous layer. This is essential for the subsequent process, inwhich Si single crystal having less defects is epitaxially developedinto a porous layer structure.

The substrate was oxidized at 400° C. in the atmosphere of oxygen forone hour. Inner walls of pores of the porous Si were covered with athermally oxidized film through the oxidization. After cleaning wasperformed with HF solution and thermal treatment was performed in theatmosphere of hydrogen, single-crystal Si epitaxially grew by 0.3 μm onthe porous Si by CVD method. The growing conditions were as follows:

source gas: SiH₄

carrier gas: H₂

temperature: 900° C.

pressure: 1×10⁻² Torr

growth rate: 3.3 nm/sec

Furthermore, 100 nm of SiO₂ layer was formed on a surface of theepitaxial Si layer by the thermal oxidization.

After the surface of the SiO₂ layer and a surface of a separatelyprepared Si substrate were overlapped and contacted with each other,thermal treatment was performed at 1180° C. for five minutes to performbonding. The porous layer was exposed to wafer edges and, instead ofetching the porous Si to some decree, water jet injection was performedwith a water pressure of 300 kgf/cm² and a diameter of 0.1 mm. Then, theporous Si layer was ruptured, the wafer was effectively divided intotwo, and the porous Si was exposed. Subsequently, the porous Si layerwas selectively etched with etching liquid of HF/H₂O₂/C₂H₅OH. The porousSi was selectively etched for a shorter time than in the third example,and completely removed. The etching rate of the non-porous Si singlecrystal to the etching liquid is remarkably low, and the etching amountin the non-porous layer provides a practically ignorable decrease offilm thickness. Specifically, a single-crystal Si layer having athickness of 0.2 μm could be formed on the oxidized Si film. Thesingle-crystal Si layer underwent no change even by the selectiveetching of the porous Si. Resulting SOI substrate was thermally treatedin the atmosphere of hydrogen.

As a result of the observation of a cross section by a transmissionelectronic microscope, it was confirmed that no crystal defect wasintroduced to the Si layer and that good crystallizability was kept.Even when no oxidized film was formed on the surface of the epitaxial Silayer, similar results were obtained. The first Si single-crystalsubstrate was used again as a first Si single-crystal substrate byremoving residual porous Si therefrom.

EXAMPLE 5

A first P-type (or N-type) (100) single-crystal Si substrate with athickness of 625 μm, a specific resistance of 0.01 Ω·cm and a diameterof eight inches was used, and anodization was performed in HF solution.A formation bath was prepared in such a manner that a sectional area ofa plane parallel with a formation electrode of an anodization layer andthe Si single-crystal base substrate was about 1.3 times an area of theSi base, and was used.

Anodization conditions are as follows:

anodization current: 2.6 A

anodization solution:HF:H₂O:C₂H₅OH=1:1:1

time: 11 minutes

The thickness of a central portion of a first porous layer of the basesubstrate subjected to the formation was about six microns, and theporosity of the central portion was about 20%. The thickness of theporous layer of a peripheral portion was about eight μm at maximum andthe porosity was 25%. Subsequent to the formation of the first layer,the formation of a second layer was performed under the followingconditions:

anodization current: 12 A

anodization solution:HF:H₂O:C₂H₅OH=1:1:1

time: one minute

For the central portion or the peripheral portion, no remarkabledifference in pore size can be found in the vicinity of the surface ofthe first porous layer. This is essential for the subsequent process, inwhich Si single crystal having less defects is epitaxially developedinto a porous layer structure.

The substrate was oxidized at 400° C. in the atmosphere of oxygen forone hour. Inner walls of pores of the porous Si were covered with athermally oxidized film through the oxidization. After cleaning wasperformed with HF solution and thermal treatment was performed in theatmosphere of hydrogen, single-crystal Si epitaxially grew by 0.3 μm onthe porous Si by CVD method. The growing conditions were as follows:

source gas: SiH₄

carrier gas: H₂

temperature: 900° C.

pressure: 1×10⁻² Torr

growth rate: 3.3 nm/sec

Furthermore, 100 nm of SiO₂ layer was formed on a surface of theepitaxial Si layer by the thermal oxidization.

After the surface of the SiO₂ layer and a surface of a separatelyprepared Si substrate were overlapped and contacted with each other,thermal treatment was performed at 1180° C. for five minutes to performbonding. The porous layer was exposed to wafer edges, and the porous Siwas etched to some degree. A multiplicity of bonded base substratesprepared as described above were simultaneously submerged in a waterbath of an ultrasonic radiation device. When about 50 kHz of ultrasonicwaves were radiated, porous Si layers of all the bonded base substrateswere ruptured, each wafer was divided into two, and the porous Si wasexposed. Subsequently, the porous Si layer was selectively etched withetching liquid of HF/H₂O₂/C₂H₅OH. The porous Si was selectively etchedin a shorter time than in the third example, and completely removed. Theetching rate of the non-porous Si single crystal to the etching liquidis remarkably low, and the etching amount in the non-porous layerprovides a practically ignorable decrease of thickness. Specifically, asingle-crystal Si layer having a thickness of 0.2 μm could be formed onthe oxidized Si film. The single-crystal Si layer underwent no changeeven by the selective etching of the porous Si.

As a result of the observation of a cross section by a transmissionelectronic microscope, it was confirmed that no crystal defect wasintroduced to the Si layer and that good crystallizability was kept.Even when no oxidized film was formed on the surface of the epitaxial Silayer, similar results were obtained. The first Si single-crystalsubstrate was used again as a first Si single-crystal substrate byremoving residual porous Si therefrom.

EXAMPLE 6

As an insulating layer, 200 nm of oxidized film (SiO₂ layer) was formedon a surface of a first single-crystal Si substrate.

A first ion implantation was performed from the surface of the firstsubstrate in such a manner that projection range fell in the Sisubstrate. In this manner, a layer serving as a separation area wasformed as a distorted layer by a microcavity layer or a layer with ahigh concentration of ions injected therein in a depth of the projectionrange. Subsequently, under substantially the same conditions as those ofthe first ion implantation, ion implantation was again performed in therange of a 10 mm wide peripheral portion of the substrate. Thereby, theion implantation amount of the peripheral portion was about twice thatof the central portion.

After the ion implantation, a surface of the SiO₂ layer and a surface ofa second Si substrate separately prepared were overlapped and contactedwith each other, then thermal treatment was performed at 600° C. toperform bonding.

While the central portion of the substrate bonded as described above washeld, and the substrate was rotated around a central axis, water jetinjection was performed from the peripheral portion in parallel with abonded face under a water pressure of 300 kgf/cm² and a diameter of 0.1mm. Then, the separation area was collapsed and the wafer was remarkablyeffectively separated.

As a result, the SiO₂ layer, surface single-crystal layer and a part ofseparation layer originally formed on the surface of the first substratewere transferred to the second substrate. A remaining part of theseparation layer was left on the surface of the first substrate. Afterthe separation, the second substrate was annealed at 1000° C.Thereafter, the separation layer transferred to the second substrate waspolished and removed with CMP device, and its surface was smoothed.

Specifically, a single-crystal Si layer having a thickness of 0.2 μmcould be formed on the oxidized Si film. For all in-plane faces, thethickness of the single-crystal Si layer formed on the insulating layeras described above was measured in 100 points or positions. As a result,the uniformity of the film thickness was 201 nm±7 nm.

As a result of the observation of a cross section by a transmissionelectronic microscope, it was confirmed that no new crystal defect wasintroduced to the Si layer and that good crystallizability was kept.

Furthermore, after thermal treatment was performed at 1100° C. inhydrogen for one hour, surface roughness was evaluated with aninteratomic force microscope. As a result, an average square roughnessin 50 μm square was about 0.2 nm, which was equivalent to the roughnessof an ordinarily marketing Si wafer.

Even when the oxidized film was formed on only the surface of the secondsubstrate or on the surfaces of the epitaxial layer and secondsubstrate, similar results were obtained.

Moreover, when the separation layer remaining on the first substrate isregenerated by etching or surface polishing, and hydrogen annealing oranother surface treatment is further applied if necessary, the substratecan be used again as a first or second substrate.

In the example, the surface area of the Si wafer is transferred to thesecond substrate via the separation layer by the ion implantation, butan epitaxial wafer may be used to transfer the epitaxial layer to thesecond substrate via the separation layer by the ion implantation.Moreover, after the ion implantation of the example, a surface SiO₂ isremoved, an epitaxial layer is formed, SiO₂ is further formed, then thebonding process is performed, so that the epitaxial layer may betransferred to the second substrate via the separation by the ionimplantation. In the latter case, the surface area of the Si wafer isalso transferred.

In the aforementioned examples for separating the composite member, evenwhen the separating force is raised, the separation proceeds toward theinside from edges of the bonded base substrates without breaking one orboth of the halfway separated bases. Moreover, the separated first basesubstrate may be used again as a semiconductor base substrate forobtaining the next SOI substrate.

Additionally, one of the separated base substrates can be reused inpreparing bonded bases, which enhances a yield of base substrateseparation.

Moreover, particles generated by the collapsed separation area can beprevented from causing contamination in the process. Furthermore, whenthe separation is performed by another method using no fluid, theseparation yield can be enhanced.

Additionally, a suitable composite member can be prepared in such amanner that when the base substrate is separated from the separationarea formed inside the bonded semiconductor base substrate or anothercomposite member, separation does not occur before the separationprocess, and separation is securely performed in the separation process.

What is claimed is:
 1. A method of separating a composite member into aplurality of members at a separation area comprising the step of: makinga mechanical strength of the separation area non-uniform and amechanical strength of a peripheral portion of the separation arealocally low by forming portions different from each other in at leastone attribute selected from the group consisting of porosity, thicknessand ion implantation amount.
 2. The method according to claim 1, whereinthe mechanical strength of said separation area is lower than that of abonded interface of said composite member.
 3. The method according toclaim 1, wherein said separation area is a porous layer formed by ananodization.
 4. The method according to claim 1, wherein said compositemember substantially has a disc shape, and the mechanical strength ofsaid separation area is high in a central portion of said compositemember, low in the peripheral portion and substantially uniform along acircumferential direction.
 5. The method according to claim 1, whereinthe porosity of said separation area is set higher in the peripheralportion than in a central portion of the separation area.
 6. The methodaccording to claim 1, wherein a porous layer is formed thicker in theperipheral portion than in a central portion of the separation area. 7.The method according to claim 1, wherein said separation area comprisesa plurality of layers different in mechanical strength.
 8. The methodaccording to claim 1, wherein said separation area comprises a layerhigh in porosity and a layer low in porosity.
 9. The method according toclaim 8, wherein the porosity of said layer high in porosity is higherin the peripheral portion than in a central portion of the separationarea.
 10. The method according to claim 8, wherein the thickness of saidlayer low in porosity is set larger in the peripheral portion than in acentral portion of the separation area, and the porosity of said layerhigh in porosity is set higher in the peripheral portion than in thecentral portion.
 11. The method according to claim 10, wherein thethickness of said layer low in porosity is set higher in the peripheralportion than in the central portion by changing a current density ofanodization in a plane.
 12. The method according to claim 11, wherein asectional area in which an ion current of an anodization liquid flows inthe vicinity of a first base substrate subjected to anodization is setlarger than an area of said first base, so that a surface density of ananodization current flowing into the peripheral portion is made higherthan the surface density of the anodization current flowing in thecentral portion, the thickness of said layer low in porosity is madelarger in the peripheral portion than in the central portion, and theporosity of said layer high in porosity formed later is made higher inthe peripheral portion than in the central portion.
 13. The methodaccording to claim 11, wherein a current guide for controlling adistribution of ion currents flowing in the surface of a first basesubstrate is provided in the vicinity of the first base substratesubjected to said anodization, so that the thickness of said layer lowin porosity is varied in the plane.
 14. The method according to claim 1,wherein said separation area is, a layer formed by ion implanting inwhich microcavities can be obtained.
 15. The method according to claim1, wherein an ion implantation amount of the peripheral portion is setlarger than that of a central portion of the separation area.
 16. Themethod according to claim 1, wherein a first base substrate is formed bymaking a single-crystal silicon substrate partially porous to form aporous single-crystal silicon layer and epitaxially developing anon-porous single-crystal silicon layer on the porous single-crystalsilicon layer.
 17. The method according to claim 16, wherein said firstbase substrate and a second base substrate are bonded through aninsulating layer, and the insulating layer is formed by oxidizing asurface of the non-porous single-crystal silicon layer of said firstbase.
 18. A method of separating a composite member comprising the stepsof: implanting ions in a predetermined depth of a first base substratecomprising a single-crystal semiconductor to obtain a separation areacontaining an ion-implanted layer; bonding said first base substrate anda second base substrate to obtain a composite member in which anion-implanted face of said first base substrate is positioned inward;and separating the composite member at a peripheral portion formed atthe separation area, a mechanical strength of the peripheral portionbeing locally low.
 19. The method according to claim 18, wherein saidion-implanted layer has a lower mechanical strength than a bondedinterface of the composite member.
 20. The method according to claim 1,wherein a difference between a porosity of said peripheral portion and aminimum value of the porosity is 5% or more.
 21. The method according toclaim 1, wherein a difference between a porosity of said peripheralportion and a minimum value of the porosity is 10% or more.
 22. Themethod according to claim 1, wherein a porosity of said peripheralportion is selected from arrange between 20% or more and 80% or less.23. The method according to claim 1, wherein a porosity of saidperipheral portion is selected from a range between 35% or more and 80%or less.
 24. The method according to claim 1, wherein a porosity of acentral portion of the separation area is selected from a range between5% or more and less than 35%.
 25. The method according to claim 1,wherein a porosity of a central portion of the separation area isselected from a range between 5% or more and less than 20%.
 26. Themethod according to claim 1, wherein said separation area comprises aportion with a high mechanical strength larger than said peripheralportion.
 27. The method according to claim 1, wherein said separationarea comprises a portion with a high mechanical strength in a positiondeviated from a center of said composite member.
 28. A method ofpreparing a semiconductor substrate using the method of separating acomposite member according to claim
 1. 29. A method of preparing asemiconductor substrate comprising, in this order, the steps of: forminga separation area having a non-uniform mechanical strength along abonding face of a first base substrate, a peripheral portion of theseparation area having a locally low mechanical strength; bonding thefirst base substrate and a second base substrate to each other to form acomposite member; and separating the composite member into a pluralityof members at the separation area.
 30. The method of preparing asemiconductor substrate according to claim 29, wherein the mechanicalstrength of said separation area is lower than that of said bondingface.
 31. The method of preparing a semiconductor substrate according toclaim 29, wherein said separation area is a porous layer formed by ananodization.
 32. The method of preparing a semiconductor substrateaccording to claim 29, wherein said composite member substantially has adisc share, and the mechanical strength of said separation area is highin a central portion of said composite member, low in the peripheralportion and substantially uniform along a circumferential direction. 33.The method of preparing a semiconductor substrate according to claim 29,wherein the porosity of said separation area is set higher in theperipheral portion than in a central portion of the separation area. 34.The method of preparing a semiconductor substrate according to claim 29,wherein said porous layer is formed thicker in the peripheral portionthan in a central portion of the separation area.
 35. The method ofpreparing a semiconductor substrate according to claim 29, wherein saidseparation area comprises a plurality of layers different in mechanicalstrength.
 36. The method of preparing a semiconductor substrateaccording to claim 29, wherein said separation area comprises a layerhigh in porosity and a layer low in porosity.
 37. The method ofpreparing a semiconductor substrate according to claim 36, wherein theporosity of said layer high in porosity is higher in the peripheralportion than in a central portion of the separation area.
 38. The methodof preparing a semiconductor substrate according to claim 36, whereinthe thickness of said layer low in porosity is set larger in theperipheral portion than in a central portion of the separation area, andthe porosity of said layer high in porosity is set higher in theperipheral portion than in the central portion.
 39. The method ofpreparing a semiconductor substrate according to claim 36, wherein thethickness of said layer low in porosity is set larger in the peripheralportion than in the central portion by changing a current density ofanodization in a plane.
 40. The method of preparing a semiconductorsubstrate according to claim 39, wherein a sectional area in which anion current of an anodization liquid flows in the vicinity of said firstbase substrate subjected to anodization is set larger than an area ofsaid first base, so that a surface density of an anodization currentflowing into the peripheral portion is made higher than the surfacedensity of the anodization current flowing in the central portion, thethickness of said layer low in porosity is made larger in the peripheralportion than in the central portion, and the porosity of said layer highin porosity formed later is made higher in the peripheral portion thanin the central portion.
 41. The method of preparing a semiconductorsubstrate according to claim 39, wherein a current guide for controllinga distribution of ion currents flowing in the surface of said first basesubstrate is provided in the vicinity of the first base substratesubjected to said anodization, so that the thickness of said layer lowin porosity is varied in the plane.
 42. The method of preparing asemiconductor substrate according to claim 29, wherein said separationarea is a layer formed by ion implanting in which microcavities can beobtained.
 43. The method of preparing a semiconductor substrateaccording to claim 29, wherein an ion implantation amount of theperipheral portion is set larger than that of a central portion of-theseparation area.
 44. The method of preparing a semiconductor substrateaccording to claim 29, wherein said first base substrate is formed bymaking a single-crystal silicon substrate partially porous to form aporous single-crystal silicon layer and epitaxially developing anon-porous single-crystal silicon layer on the porous single-crystalsilicon layer.
 45. The method of preparing a semiconductor substrateaccording to claim 44, wherein said first base substrate and said secondbase substrate are bonded through an insulating layer, and theinsulating layer is formed by oxidizing a surface of the non-poroussingle-crystal silicon layer of said first base.
 46. A method ofpreparing a semiconductor substrate comprising the steps of: implantingions in a predetermined depth of a first base substrate comprising asingle-crystal semiconductor to obtain a separation area containing anion-implanted layer; bonding said first base substrate and a second basesubstrate to obtain a composite member in which an ion-implanted face ofsaid first base substrate is positioned inward; and separating thecomposite member at a peripheral portion formed at the separation area,a mechanical strength of the peripheral portion being locally low. 47.The method of preparing a semiconductor substrate according to claim 46,wherein said ion-implanted layer has a lower mechanical strength than abonded face of the composite member.
 48. The method according to claim29, wherein a difference between a porosity of said peripheral portionand a minimum value of the porosity is 5% or more.
 49. The methodaccording to claim 29, wherein a difference between a porosity of saidperipheral portion and a minimum value of the porosity is 10% or more.50. The method according to claim 29, wherein a porosity of saidperipheral portion is selected from a range between 20% or more and 80%or less.
 51. The method according to claim 29, wherein a porosity ofsaid peripheral portion is selected from a range between 35% or more and80% or less.
 52. The method according to claim 29, wherein a porosity ofa central portion of the separation area is selected from a rangebetween 5% or more and less than 35%.
 53. The method according to claim29, wherein a porosity of a central portion of the separation area isselected from a range between 5% or more and less than 20%.
 54. Themethod according to claim 29, wherein said separation area comprises aportion with a high mechanical strength larger than said peripheralportion.
 55. The method according to claim 29, wherein said separationarea comprises a portion with a high mechanical strength in a positiondeviated from a center of said composite member.
 56. A separation methodcomprising the steps of: implanting ions in a predetermined depth of afirst base substrate comprising a single-crystal semiconductor in amanner such that an ion implantation amount is non-uniform along asurface of the first base substrate to obtain an ion-implanted layer;bonding the first base substrate and a second base substrate to obtain acomposite member in which an ion-implanted face of the first basesubstrate is positioned inward; and separating the composite member at aperipheral portion of the ion-implanted layer, the ion implantationamount of which peripheral portion is locally large.
 57. A method ofpreparing a semiconductor substrate comprising the steps of: implantingions in a predetermined depth of a first base substrate comprising asingle-crystal semiconductor in a manner such that an ion implantationamount is non-uniform along a surface of the first base substrate toobtain an ion-implanted layer; bonding the first base substrate and asecond base substrate to obtain a composite member in which anion-implanted face of the first base substrate is positioned inward; andseparating the composite member at a peripheral portion of theion-implanted layer, the ion implantation amount of which peripheralportion is locally large.